The band-gap voltage reference circuit is well-known to the designers of analog circuits. This reference circuit provides a constant voltage as independent as possible of the environmental temperature at which the circuit operates. This type of circuit is present in many systems manufactured with integrated circuits. For example, a constant voltage reference is required for analog/digital converters. Such converters compare the value of a voltage reference signal against the value of samples to be converted.
Numerous circuit arrangements are known which provide a band-gap voltage reference of the time-continuous type (i.e., of the type which can be used for 100% of the time). One such circuit is disclosed in Gray and Mayer, "Analysis and Design of Analog Integrated Circuits", published by John Wiley & Sons. These circuits are generally manufactured using bipolar junction technology because their operating principle is based on intrinsic properties of bipolar junction transistor (BJT) as described in the above noted publication.
The operating principle of a band-gap voltage reference circuit is based on the compensating increases and decreases in the rate of voltage change due to changes in environmental temperature. That is, the voltage between the base and the emitter of one bipolar transistor decreases with the environmental temperature at the rate of approximately 2 mV/.degree. C.
In the annexed FIG. 1 a band-gap circuit known as Brokaw cell is shown as an example.
Such a Brokaw cell comprises a transistor TI and a transistor T2 of the type pnp connected together in a current mirror configuration. The emitters of the transistors TI and T2 are connected to a reference of a supply voltage Vcc. The bases of the transistors TI, T2 are connected together.
The base and collector of the transistor TI are connected together and to the collector of a transistor T3 of the type npn.
More particularly, the collectors of the transistors TI, T2 are connected to respective collectors of two transistors T3 and T4 having different emitter areas, the emitter area of the transistor T4 being n times greater than the emitter area of the transistor T3. A voltage divider formed by two resistors R1, R2 is connected between the emitter of transistor T4 and the ground with the connection node between the resistances being connected to the emitter of transistor T3.
The band-gap voltage Vbg is taken from the interconnection node between the bases of the two transistors T3 and T4 and it is given by the following relation: EQU Vbg=Vbe+K*VT, (1)
where Vbe is the voltage drop between base and emitter, VT is the threshold voltage and K is a design constant having the following value: EQU K=2*ln(n)*R2/R1. (2)
There is a dependency from temperature of the voltage drop Vbe which may be defined as follows: EQU Vbe(T)=Vgo+.alpha.*T+f(T.sup.2), (3)
where Vgo is the silicon band-gap voltage at zero Kelvin degrees.
If we use the relation (3) inside relation (1) we obtain that: EQU Vbg=Vgo+.alpha.*T+f(T.sup.2)+K*VT. (4)
Since the variation with temperature of the threshold voltage VT is known and the constant K value may be designed, it's possible to reduce to zero the first order dependence from temperature of the band-gap voltage Vbg.
FIG. 2 shows a diagram of the variations versus temperature of the band-gap voltage Vbg for a known Brokaw cell.
As may be appreciated, on a temperature range from -50 up to 150.degree. C., a 1.5 mV variation of the band-gap voltage may be observed. The gaussian curvature which is reported in FIG. 2 is due just to the second order effects dependence which has been indicated in relation (4).
This variation is in addition to errors due to the manufacturing process spread, to the circuit components mismatch, to mechanical stress during the packaging phase. Therefore, the probability of failing to meet the values provided by the design specifics is high.
As a matter of fact, second order effects must be compensated to comply with the design specifics of the voltage regulator.
The prior art proposes some solutions to meet this requirement. Known prior art solutions are based on the fact that the dependence on temperature of the base-emitter voltage drop may be modified by biasing the transistor with an absolute temperature proportional current (PTAT voltage regulators).
However, those solutions present some drawbacks:
the corresponding regulator circuits are power consuming when there are many current branches. Moreover, a possible mismatch between the current mirrors reduce the compensation efficiency. See for instance: Gunawan and others: "A Curvature-Corrected Low Voltage Band-gap Reference"--IEEE Journal of Solid State Circuit, vol. 28, No. 6, June 1993. PA1 known regulators do not work correctly when supplied with voltage values less than 3 V, which are the supply voltage values now required in many microelectronics applications. See in this respect: Meijer and others: "A New Curvature-Corrected Band-gap Reference"--IEEE Journal of Solid State Circuit, vol. SC-17, No. 6, December 1992.